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Meteorological and dust aerosol conditions over the
western Saharan region observed at Fennec Supersite-2
during the intensive observation period in June 2011
M. C. Todd, C. J. T. Allen, M. Bart, M. Bechir, J. Bentefouet, B. J. Brooks,
C. Cavazos-Guerra, T. Clovis, S. Deyane, M. Dieh, et al.
To cite this version:
M. C. Todd, C. J. T. Allen, M. Bart, M. Bechir, J. Bentefouet, et al.. Meteorological and dust aerosol conditions over the western Saharan region observed at Fennec Supersite-2 during the intensive ob-servation period in June 2011. Journal of Geophysical Research: Atmospheres, American Geophysical Union, 2013, 118 (15), pp.8426-8447. �10.1002/jgrd.50470�. �hal-00829054�
Meteorological and dust aerosol conditions over the western Saharan
region observed at Fennec Supersite-2 during the intensive
observation period in June 2011
M. C. Todd,1,2C. J. T Allen,3M. Bart,4M. Bechir,5J. Bentefouet,6B. J. Brooks,7 C. Cavazos-Guerra,1T. Clovis,6S. Deyane,5M. Dieh,5S. Engelstaedter,3C. Flamant,8 L. Garcia-Carreras,4A. Gandega,5M. Gascoyne,4M. Hobby,4C. Kocha,8C. Lavaysse,8 J. H. Marsham,4,7J. V. Martins,9J. B. McQuaid,4J. B. Ngamini,10D. J. Parker,4 T. Podvin,11A. Rocha-Lima,9S. Traore,5Y. Wang,1and R. Washington3
Received 23 October 2012; revised 2 May 2013; accepted 4 May 2013; published 15 August 2013.
[1] The climate of the Sahara is relatively poorly observed and understood, leading to errors in forecast model simulations. We describe observations from the Fennec Supersite-2 (SS2) at Zouerate, Mauritania during the June 2011 Fennec Intensive Observation Period. These provide an improved basis for understanding and evaluating processes, models, and remote sensing. Conditions during June 2011 show a marked distinction between: (i) a “Maritime phase” during the early part of the month when the western sector of the Sahara experienced cool northwesterly maritime flow throughout the lower troposphere with shallow daytime boundary layers, very little dust uplift/transport or cloud cover. (ii) A subsequent “heat low” phase which coincided with a marked and rapid westward shift in the Saharan heat low towards its mid-summer climatological position and advection of a deep hot, dusty air layer from the central Sahara (the “Saharan residual layer”). This transition affected the entire western-central Sahara. Dust advected over SS2 was primarily from episodic low-level jet (LLJ)-generated emission in the northeasterly flow around surface troughs. Unlike Fennec SS1, SS2 does not often experience cold pools from moist convection and associated dust emissions. The diurnal evolution at SS2 is strongly influenced by the Atlantic inflow (AI), a northwesterly flow of shallow, cool and moist air propagating overnight from coastal West Africa to reach SS2 in the early hours. The AI cools and moistens the western Saharan and weakens the nocturnal LLJ, limiting its dust-raising potential. We quantify the ventilation and moistening of the western flank of the Sahara by (i) the large-scale flow and (ii) the regular nocturnal AI and LLJ mesoscale processes.
Citation: Todd, M. C., et al. (2013), Meteorological and dust aerosol conditions over the western Saharan region observed at Fennec Supersite-2 during the intensive observation period in June 2011,J. Geophys. Res. Atmos., 118, 8426–8447, doi:10.1002/jgrd.50470.
1. Introduction
1.1. The Nature of the Saharan Climate System [2] The atmospheric boundary layer over the Sahara (SABL) is a unique and important feature of the global cli-mate system, exhibiting numerous extremes. During boreal summer, in particular, intense surface heating produces the deepest boundary layers on Earth of up to 6 km [Cuesta et al., 2009], and the associated development of the Saharan heat low (SHL), over a huge, largely uninhabited expanse of northern Mali, southern Algeria, and Mauritania (Figure 1). The SHL is a result of the development of a likely dome-shaped thermal heating feature in the lower tropo-sphere below ~700 hpa and is essentially the summertime position of the wider West African heat low (WAHL). The SHL plays a pivotal role in the West African Monsoon
This article is a companion to Marsham et al. [2013] doi:10.1002/jgrd.50211. 1University of Sussex, UK.
2
Now at Aeroqual Limited, Auckland, New Zealand. 3University of Oxford, UK.
4
School of Earth and Environment, University of Leeds, UK. 5Office National de Météorologie, Mauritanie.
6
AeroEquip & Conseil, Douala, Cameroon.
7National Center for Atmospheric Science, University of Leeds, UK. 8
Laboratoire Atmosphères, Milieux, Observations Spatiales, CNRS and Université Pierre et Marie Curie, Paris, France.
9
University of Maryland, Baltimore County (UMBC), USA. 10ASECNA, Dakar-Yoff, Senegal.
11
LOA, University of Lille, France.
Corresponding author: M. C. Todd, University of Sussex, UK. (m. todd@sussex.ac.uk)
©2013. American Geophysical Union. All Rights Reserved. 2169-897X/13/10.1002/jgrd.50470
[e.g., Sultan et al., 2003]. Further, this region is also where the highest vertically integrated dust aerosol loading any-where in the Earth’s atmosphere is found. Several satellite-derived aerosol products show that the SHL is co-located with the aerosol maximum during the boreal summer [Engelstaedter et al., 2006] (Figure 1). Saharan dust aerosols have a complex interaction with the regional and global climate through: (i) The direct effect of dust aerosols on the shortwave and longwave radiative flux, which can exceed many tens Wm 2at the TOA and in excess of 100 WM 2 at the surface [Haywood and Boucher, 2000]. (ii) The indi-rect effect of dust aerosols on cloud microphysical processes and properties [e.g., Kaufman et al., 2005]. (iii) The semi-direct effect of dust generated by absorptional heating within dust layers, which changes relative humidity and vertical sta-bility [Dunion and Velden, 2004]. (iv) Atmospheric chemical processes on aerosol surfaces [Cwiertny et al., 2008]. (v) The influence of Saharan dust deposition on terrestrial and marine bio-geochemical cycles [e.g., Jickells et al., 2005; Mahowald et al., 2010]. A wide range of meteorological dust generation mechanisms is responsible for this aerosol“hotspot” over the SHL region (see Knippertz and Todd [2012] for a review).
[3] However, the central Sahara, with no significant popu-lation centers, suffers from a particularly sparse observa-tional network, which hinders our understanding of the climate, with profound consequences for weather forecasting and climate prediction in the region and beyond. It should be noted that the SHL characteristics as described by reanalyses
data like those in Figure 1 may be subject to considerable un-certainty given the lack of data for assimilation into the model. Haywood et al. [2005] and Milton et al. [2008] showed large biases due to dust processes in Numerical Weather Prediction model outgoing longwave radiation (of ~50 Wm 2) and surface net radiation (of ~100 Wm 2), respectively. Such errors in radiation lead to errors in key dynamical features beyond the Sahara (e.g., the African Easterly Jet), with consequences for tropical cyclone development in the Atlantic and the circulation over the Mediterranean basin. It has been shown that including effects of dust aerosols into numerical models has a beneficial impact on weather and climate simulations [Perez et al., 2006; Rodwell and Jung, 2008; Tompkins et al., 2005]. 1.2. The Project“Fennec: The Saharan Climate System”
[4] In the last decade or so, a number offield campaigns have been undertaken to improve our baseline data on mete-orological and aerosol conditions in the wider North African sector (Figure 1). These include Jet2000 [Thorncroft et al., 2003], SHADE in 2001 [Haywood et al., 2003], the Bodélé Dust Experiment (BoDEx) in 2005 [Todd et al., 2007; Washington et al., 2006], the African Monsoon Multidisciplinary Analysis (AMMA) in 2006 [Redelsperger et al., 2006], the Dust and Biomass Experiment DABEX in 2006 [Haywood et al., 2008], Dust Outflow and Deposition to the Ocean in 2006 [McConnell et al., 2008], the Geostationary Earth Radiation Budget Intercomparison of Long-wave and Short-wave radiation in 2007 [Haywood et al., 2011], and the Saharan Mineral Dust Experiment in 2006 and 2008 [Ansmann et al., 2011]. With the exception of limited measurements during AMMA [Cuesta et al. 2008; Messager et al., 2010], no previous campaign has focused on the SHL region of North Africa during the sum-mer dust season (Figure 1).
[5] Fennec is an international consortium with research groups in the UK, France, Germany, and the USA working in collaboration with the Meteorological Services of Algeria, Mauritania in North Africa, and the European partner coun-tries. Fennec was conceived and designed tofill critical gaps in observations and understanding of the Saharan climate sys-tem including the role of mineral dust aerosol. To this end, Fennec involved a large-scale, multi-platform, surface, and airborne observational campaign in the SHL region over an extended period during 2011–2012 to quantify the physical processes controlling the Saharan climate system. These unique observations are then used in combination with opera-tional and research-oriented numerical models to analyze aero-sol, radiative, land surface, thermodynamic, and dynamic processes over the Sahara to evaluate and attribute errors in weather and climate models for this region.
[6] The ground-basedfield campaign is a cornerstone of Fennec and has three components: high temporal resolution, multi-parameter observations of the atmosphere at two main supersites, with SS1 in Southern Algeria and Supersite-2 (SS2) in Northern Mauritania (Figure 1), and less compre-hensive observations of surface meteorology from a network of remote automatic stations distributed across the SHL region [Hobby et al., 2012]. The SSs are strategically located as close as possible to the heart of the SHL/aerosol maximum as logistics allowed. SS2 is located to the west of the mean SHL and aerosol maximum and as such is indicative of Figure 1. The Fennec domain and climatology. Figure
shows mean (2000–2012) June–Sept. aerosol optical depth (AOD) from satellite MISR data (shaded, contour intervals are 0.4, 0.6, and 0.8) and key mean June–September circula-tion features derived from ERA-Interim reanalysis data (1979–2012), specifically the mean 925 hPa winds (arrows); the mean position of the Saharan heat low core (1008 hPa con-tour of sea level pressure, thick red concon-tour); the mean position of the inter-tropical discontinuity (solid blue line, as defined by the 10 gkg–1 contour of 925 hPa specific humidity). Figure also highlights the location of the two supersites (SS1 yellow square, SS2 yellow circle), the Bir Moghrein sampling station (yellow cross) and approximate aircraft flight zone (green polygon). Also indicated are surface elevation (dashed cyan contour, 1000, 1500, and 2000 m) and the approximate loca-tion of recentfield campaigns.
conditions over the broad western Saharan region. This pa-per, and the companion paper for SS1 by Marsham et al. [2012], aims to provide an overview of the surface observa-tions during Fennec. Here, we focus specifically on the ob-servational program at the Fennec SS2. We present analysis of results from the ground-based instruments, contextualized with analyses from Fennec research aircraft observations, reanalysis, satellite sources, and numerical models where ap-propriate. Section 2 describes the instrument characteristics. Section 3 presents our results, in which we focus on the synoptic-seasonal-scale evolution of the atmospheric struc-ture over western Saharan region (section 3.1) and SS2 (sec-tion 3.2), a focus on two specific components of the lower-level circulation, namely the Atlantic inflow (AI) (section 3.3) and the low-level jet (LLJ) (section 3.4) which play im-portant roles in the heat and moisture budgets of the western Saharan region and proximate SHL. We then consider dust aerosol conditions at SS2 in this context (section 3.5) before summarizing the mean diurnal cycle (section 3.5). Discussion and conclusion are presented in section 4.
2. The Instruments Deployed at Fennec SS2 and Additional Data Used
[7] Fennec SS2 is located in northern Mauritania near the town of Zouerate at 22.75N, 12.48W and an elevation of 343 m above sea level (Figure 1). This location was selected as the nearest location on the westernflank of the SHL/aero-sol maximum with a reliable infrastructure. It is also relatively close to the Fennec aircraft campaign base at Fuerteventura in the Canary Island (Figure 1) allowing over-flights to support radiation budget closure experiments. Fennec SS1, in southern Algeria is located towards the center/eastern flank of the mean SHL. The Fennec instru-ments at SS2 were deployed by the Office National de Météorologie (ONM) in Mauritania at the end of May 2011 in the environs of the airport at Zouerate close to the existing World Meteorological Organisation (WMO) station 61404. A suite of meteorological and aerosol instruments were deployed at SS2-Zouerate (Table 1) to measure key meteoro-logical and aerosol quantities. The core instruments of the flux tower and Cimel sun photometer instrument were
deployed for the Extended Observation period of one-year duration from June 2011 to June 2012. Additional instru-ments to provide greater detail on the vertical structure of the SABL were deployed for the shorter intensive observa-tion period (IOP) from 1 to 30 June 2011. In addiobserva-tion, during the IOP, an aerosol sampling instrument was deployed some 290 km to the north of SS2 at the remote town of Bir Moghrein 25.23N, 11.62W (Figure 1). This location was selected to ensure no contamination of physical aerosol sam-ples from the mining activities at Zouerate. Instruments were deployed and operated by the ONM Mauritanie. The charac-teristics of the instruments are described below.
[8] The Fennec flux tower consisted of 15 m high mast instrumented with 20 Hz sonic anemometers at 10 and 15 m. At 2 m, there were passively ventilated measurements of temperature and humidity, as well as pressure. A separate 2 m mast was used to measure upwelling and downwelling solar and longwave radiativefluxes, and ground heat flux at 10 cm. Radiosondes were launched three times daily at 00:00, 06:00, and 12:00 UTC on all days during the IOP except on 25–30, when four launches per day were made at 00:00, 06:00, 09:00, and 12:00 UTC. SS2 local time is the same as UTC. Afternoon soundings were not made, due to a limited number of trained staff, working one shift per day. Reducing sampling occurred on 26 (06:00 and 12:00 UTC only), 13 (00 and 12:00 UTC), 2 and 4 (00:00 UTC only), and 5 (18:00 UTC only). There were no observations made on 1 and 3 June 2011. To increase sampling, we make use of both the radiosonde ascent and descent profiles, which covers a period of around 90 min, although vertical resolu-tion from the descent is reduced. We present analysis of indi-vidual ascents and of data interpolated to a grid with 20 m vertical resolution and 1-hourly temporal resolution over the period 00:00 to 14:00 UTC during which time observa-tions were most frequent. No interpolation is made during the mid-afternoon-evening period when data are absent.
[9] Pilot balloons (hereafter“pibals”) were released twice daily at 0900 and 1700 UTC and used to derive wind speed throughout their ascent, using a single theodolite method. These data complement the radiosonde and sodar wind mea-surements. The vertical extent of pibal wind estimates varied partly as a function of aerosol and wind conditions between Table 1. Instruments Deployed at SS2 During Fennec IOP 2011
Instrument Deployment Sampling Frequency Manufacturer Notes
Radiosonde 1–30 June Three times daily at 00:00, 06:00, and 12:00 UTC
Vaisala RS92 GPS No data on: 1 and 3; 06 and 12 UTC on 2 and 4;
06 UTC on 13; 00 UTC on 24. 5 June 18 UTC
only. 25–30 additional data at 09 UTC.
Sodar (1650–2750 Hz) 1–30 June 10 min Scintec Problems with overheating through
the IOP Pilot balloons (PIBAL) 11–30 June Twice daily at 09.00 and
17:00 UTC
Flux tower 1–30 June 10 min (from 10 Hz data) University of Leeds; Metek USA-1 sonic anemometers; E-pulse ee04 humidity sensor, Hukseflux ground
heat-flux sensor; Kipp & Zonen CNR4 radiometers
Substantial data loss after 20 June
Sun photometer 1–30 June 15 min Cimel C-138
LACO Aerosol
Sampling Station– (LASS)
23 May–26 June Average 6 hourly Laboratory for Aerosol, Clouds, and Optics (LACO) at the University of
~500 m and greater than 10,000 m. Vertically integrated aerosol conditions were observed using a Cimel C-318 sun photometer, installed as part of the AErosol RObotic NETwork (AERONET) [Holben et al., 1998]. A Scintec MFAS phased array sodar was deployed, and the vertical profile of wind speed and direction was retrieved at a nominal 10-min temporal and 10-m vertical resolution. Although the sodar was identical to that at SS1, the sampling rate in the afternoon however was often poor due to instrument overheating. Due to these problems, the results from sodar require considerable post-processing and are not included in this paper.
[10] Some dropsonde measurements from researchflights are also discussed. Flights with the French F-20 and the British BAe146 aircraft were made: the FF22 flight (F-20 aircraft) and various BAe146 flights (with labels such as B607 and B608) are used here. Both aircraft used the Vaisala RD93 sonde.
[11] In addition, in situ aerosol samples were collected at Bir Moghrein using the Laboratory for Aerosol Clouds and Optics (LACO) Aerosol Sampling Station (LASS). LASS is an automatic system for collection of aerosol particles on filters designed and built by LACO at the University of Maryland Baltimore County, USA. The station has a car-tridge with eight sampling positions for Nuclepore filters (with one of the positions reserved for reference blank filters); each position is separated in two stages for the collec-tion offine (nominal aerodynamic diameter d < 1.5 mm) and coarse (1.5mm < d < 10 mm) particles. The station PM10 inlet and filter cartridges were installed at a height of 3 m a.g.l. Each one of the eight sampling positions in the cartridge is connected individually through vacuum tubes to the automatic control and vacuum system at the ground level. LASS was operational from 23 May to 26 June, collecting between three and four filters daily between 07:00 and 13:00 UTC, 13:00 and 19:00 UTC, 19:00 and 21:00 UTC, and 21:00 and 07:00 UTC. Thefine and coarse Nuclepore filters collected by LASS will be submitted to gravimetric analysis, elemental composition, optical re flec-tance techniques, and electron microscopy for determina-tion of size distribudetermina-tion and shape of the particles. Preliminary samples have been analyzed for a selection of samples discussed here.
[12] The Fennec observational data set was also complemented with contextual information from (i) ERA-Interim global atmospheric reanalysis fields [Dee et al., 2011] produced by the European Center for Medium-Range Weather Forecasts (ECMWF). We use the high-resolution
0.5 horizontal grid and 60 model sigma-level data. (ii) The ECMWF operational analyses at 1.125 which have 91 model levels. In addition, we supplement Fennec observa-tions with SYNOP reports from the WMO station 61640 at Zouerate reporting at 06:00, 09:00, 12:00, 15:00, and 18:00 UTC daily. A simulation using the Weather Research and Forecasting (WRF) mesoscale model was conducted for the period 11–19 June 2011 with a configuration summarized in Table 2. We also utilize information of dust aerosol distribu-tion from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instrument on the geostationary Meteosat Second Generation (MSG) satellites. Based on infrared chan-nels, a false-color dust product has been developed described by Lensky and Rosenfeld [2008], in which dust appears in pink tones as a result of contrast between the 12.0 and 10.8 um channels and high emissivity of dust in the 8.7 um channel.
3. Results and Discussion
3.1. Synoptic Overview of the IOP at SS2
[13] Here we consider the synoptic-scale structure of the atmosphere in the North African sector during June 2011. In many key components of the large-scale circulation rele-vant to conditions at SS2, a distinction can be made between two periods within the IOP. These roughly correspond to an early period from 2–12 June and a latter period from 13 to the end of the month. These phases are equally pertinent to the conditions at SS1 in southern Algeria, as described in the companion paper of Marsham et al. [2012]. With respect to conditions at SS2, we refer to these periods as the “Maritime phase” and “heat low phase”, respectively, and described in more detail in section 3.2. During the Maritime phase, the upper level pattern exhibited a trough centered over the Iberian Peninsula extending southwards over the northern extremity of North Africa, driving mid and upper level anomalous westerlies over northwest Africa and SS2 (Figure 2a). Subsequently, over the latter heat low phase of 13–25, June, anomalous positive geopotential heights dominated over Iberia and the extremity of northwest Africa, associated with the passage of three upper level ridges, driving an anomalous mid and upper level easterly flow, with absolute easterlies at lower levels, over the west-ern Saharan sector and SS2 (Figure 2b).
[14] The summertime low-level circulation over western North Africa is dominated by the climatological features of the Azores high and the SHL. During the Maritime phase, the Azores high ridged towards the coast of northwest Africa and moved westward subsequently (not shown). The SHL has a pronounced seasonal cycle [Lavaysse et al., 2009] involving a southeast to northwest migration from its position to the south of the Hoggar mountains (~18N, 5E) in May to its most northerly position close to 24N and 0W during July and August. The mean date of transition between these two states is 20 June [Lavaysse et al., 2009]. In 2011 the transition occurred on 11 June, associated with the maritime-heat low phase transition. During early June 2011 (Figure 3a), the SHL remained relatively stationary in an anomalously eastern location (~15E), similar to the mean state for May with strong northwesterly inflow of maritime air over much of the western Saharan region. From 13 June, the SHL exhibited an abrupt westward displacement (to ~5–10W), with two distinct intraseasonal pulses. As Table 2. WRF Configuration
Model Name WRF
Reference Michalakes et al. [2005]
Boundary conditions ERA-Interim reanalyses Domain Inner nest: 18.5–27N, 20W–8W Horizontal grid spacing 9 km outer domain and 3 km inner nest.
Vertical levels 60
Simulation period 10–19 June 2011
Height of lowest layer 10 m
Physical schemes Microphysics: Lin et al. 2003 scheme PBL: Mellor-Yamada-Janjic TKE scheme (MYJ)
Surface: Unified Noah land-surface model. Cumulus convection: Grell-3D
such, the Maritime phase (heat low phase) experienced an eastward (westward) displaced SHL with the northwestward shift (although operating at intraseasonal timescales), occur-ring in the middle of June 2011, broadly consistent with the mean seasonal cycle and “transition” to the summertime state of the SHL during late June. These two phases are broadly congruent with the“east” and “west” phases of the intraseasonal heat low mode of variability described from statistical analysis of reanalysis data by Chauvin et al. [2010] and the conceptual modes of the AI described by Grams et al. [2010]. In the Maritime phase (“heat low east”), the Sahara is effectively“ventilated” by cool advection from the Atlantic sector restricting the heat low to the central/east-ern Sahara. In contrast, during the heat low phase (“heat low west”), the Sahara is “ventilated” substantially by cool advection over Libya and the SHL shifts to the western
Sahara. Chauvin et al. [2010] identify the southward penetra-tion of Rossby wave disturbances over Europe and North Africa as the likely driver of phase shifts.
[15] The westward propagation of monsoon troughs embedded within African Easterly Waves (AEW) is known to be important for various dust emission processes [Knippertz and Todd, 2010]. During the Maritime phase, the AEW activity over continental North Africa was suppressed. From around 13 June onwards during the heat low phase, a sequence of westward propagating troughs is apparent (Figure 3a). As a result, the Maritime phase experienced exten-sive northwesterly flow across the western Saharan region sector. In contrast, the heat low phase of Fennec IOP 2011 exhibited a strong easterly and northeasterly synoptic-scale flow over central Algeria, northern Mali, and Northern Mauritania, around the northern flank of the heat low and Figure 2. Circulation during the“maritime” and “heat low” phases of Fennec IOP. (a) Mean anomalies of
300 hPa geopotential height and absolute 925 hPa winds from ERA-Interim data during 1–11 June 2011. (b) As Figure 2a but for 13–25 June 2011.
AEW troughs (Figures 3a and 3b, respectively). These structures strongly influenced the local meteorology and aero-sol conditions observed at SS2 during the Fennec IOP, as explained below.
3.2. Characteristic Atmospheric Structure at SS2 During“Maritime” and “Heat Low” Phases
[16] The influence of the large-scale circulation associated with the Maritime and heat low phases is readily apparent in the observations at SS2. Radiosondes (Figure 4a) show the Maritime phase is characterized by
[17] 1. A cool, dry lower troposphere below about 500 hPa, in which westerlies extend from the top of the convective boundary layer (CBL) throughout the troposphere, providing large-scale advective cooling of the Saharan atmosphere.
[18] 2. Development of a moist daytime CBL up to ~880 hPa at 12:00 UTC (Figure 4a red line) with a ~5K potential temperature inversion“lid” above the CBL.
[19] 3. Relatively weak nocturnal surface temperature inversion at 00:00 UTC (Figure 4a green line) (but not evident at 06:00 UTC, yellow line in Figure 4a, as explained in section 3.4)
[20] In contrast, the heat low phase (Figure 4b) is charac-terized by
[21] 1. A deep, dry almost dry-adiabatic layer from the top of the CBL at ~850 hPa to the free troposphere at ~550 hPa, namely the Saharan residual layer (SRL), advected from the central Sahara (based on samples of back-trajectory analyses (not shown)). Potential temperatures at 800 hPa are ~10K higher than during the Maritime phase.
[22] 2. Maximum relative humidity values near the top of the SRL are frequently close to values for shallow convective cloud formation.
[23] 3. A cool, moist CBL below the SRL, which develops through the day (Figure 4b red line). By the end of the month cases where an almost “pure” well-mixed SABL occur, where temperatures are near dry-adiabatic from the surface to the top of the SRL (Figure 4b red line).
[24] Both phases experience a nocturnal, shallow, mixed relatively cool, moist near surface layer, with north-northwesterlyflow (evident at 06:00UTC in the yellow lines in Figures 4a and 4b), indicative of AI (see section 3.3.) and a typically northeasterly LLJ above (see section 3.4). The LLJ maximum is in the inversion capping the stable nocturnal boundary layer at 00:00 UTC or the AI at 06:00 UTC.
[25] The transition from the maritime to heat low phases over the IOP is illustrated in the time series from radiosondes (Figure 5) and surface observations (Figure 6). Profiles of potential temperature (Figure 5a) show a distinction between the cool maritime lower tropospheric conditions with little evidence of a SRL before 14 June, and the hotter well-mixed SRL, which is well developed from 20 June onwards. The strong mid-tropospheric westerlies are apparent in the Maritime phase followed by slacker easterlies (Figures 5d and 5e). The CBL at 12:00 UTC is generally slightly deeper during the heat low phase, up to ~880 hPa (Figure 4b red line) compared to the Maritime phase, up to ~850 hPa (Figure 4a red line). Typically boundary layer heights (calculated as the height at which the potential temperature is 0.5 K greater than that at 930 hPa, following Marsham et al. [2012]) at the time of last daily observation (~13.30TC) are quite low for the Sahara; ~1500 m in the heat low phase and about 1000 m in the Maritime phase, but in some cases towards the end of the heat low phase, the sonde descent does indicate a nearly well-mixed boundary layer up to 5000 m in height (Figure 4b red line). Clearly, however, we do not sample the full extent of afternoon boundary-layer development. Nearly well-mixed boundary layers up to ~5 km, height, respectively, are observed from dropsondes released from the Fennec research aircraft flights at locations 250 km northeast of SS2 during the mid-late afternoon periods on 21 and 22 June (Figure 4c). It is interesting to note that the contrasting conditions between the Maritime phase and heat low phase are evident in data from SS1 [Marsham et al., 2012], where, despite SS1’s continental location, CBL depths of ~2 km and ~5 km, respectively, were observed.
Figure 3. The low-level circulation during Fennec IOP. (a) Time-longitude plot of absolute SHL intensity anomalies (defined as the thickness of the 925–700 hPa layer from Lavaysse et al. [2009]) averaged between 20 and 25N. (b) Time-longitude plot of 850 hPa meridional winds (shaded) averaged over 20–25N, with 925 hPa specific humidity (0.01 gkg 1contour) and 925 hPa winds at latitude of SS2 (22.75N). Thick black line denotes location of example AEW trough. Note different longitudinal range in the Figures 3a and 3b.
[26] Low-level moisture (Figure 5b, typically around 10 gkg 1) is concentrated in the lowest ~700 m a.g.l. with deeper moist layers during the Maritime phase, and more variable conditions during the heat low phase, associated with periods of hot, dry advection from the central Sahara. Moistureflux (Figure 5f) is consistent over the period and is dominated by moistureflux in the LLJ and AI (see sections 3.3. and 3.4), clearly indicating the nocturnal moistening of the western Saharan sector, for even with a northeasterly
LLJ, the moisture flux typically re-circulates towards the Sahara further south of SS2 (Figure 2b). Mid-tropospheric moisture (Figure 5b) is higher (>5 gkg 1at 3000 m) during the heat low phase compared to the Maritime phase (<1 gkg 1) indicating greater vertical mixing during the daytime of low-level moisture which was advected into the SHL during the previous night. The Maritime phase after 3 June is substan-tially cloud-free over SS2 (Figure 6f) and indeed much of the western Saharan sector of Mauritania, Northern Mali, Figure 4. (a) Tephigrams from radiosondes at SS2 characteristic of (a) the IOP Maritime phase on 6 June
and (b) the heat low phase on 29 June. Green, yellow, and red lines are from the 00:00, 06:00, and 12:00 UTC, respectively. Wind barbs are shown for the 06:00 UTC radiosonde only. (c) Tephigram fromflight B608 dropsonde at 18:55 UTC on 22 June at 23.5N, 9.2W.
Figure 5. Time series at SS2 of the vertical profile of (a) potential temperature (K), (b) WVMR (gkg 1), (c) relative humidity (%), (d) wind speed (ms 1), (e) wind direction (degrees from north), and (f) WVMR flux (gkg 1
and Western Algeria (Figure 7a). During the heat low phase at SS2, high relative humidity (Figure 5c) is observed at the top of the SABL close to 5000 m, sufficient for shallow convective cloud development during the evening (Figure 6f), apparent too over the wider western Saharan domain (e.g., Figure 7f). In the south of the western Saharan domain during the heat low phase, mesoscale convective system (MCS) activity is ob-served on some days to the south of SS2, e.g., 20 June (Figure 7d) associated with moist monsoon surges in the in AEW trough (Figure 3b). Atmospheric aerosol loadings (Figure 6g, pink coloring in Figure 7) are much higher during the heat low phase compared to the Maritime phase, as discussed in detail in section 3.5.
[27] The surface daytime maximum temperatures rise from the low 30sC in the early phase to low 40sC by the end of the IOP, while the minima rise from the high 10sC to the high 20s C (Figure 6a). Moreover, despite greater cloud cover, the magnitude of the nocturnal near-surface tempera-ture inversions is far higher in the heat low phase (compare green/yellow lines in Figure 4b with Figure 4a) such that nocturnal decoupling of the surface from near-surface
conditions is greater. Water vapor mixing ratios (WVMR) observations are sparse after the middle of the month, but SYNOP dew-point temperatures (Figure 6a) indicate pro-nounced variability during the heat low phase, with particu-larly dry conditions on 22–24 and 29–30 June when Td values<0C are observed. Whilst SS2 lies too far north to be frequently under the influence of the monsoon flow, the passage of AEW troughs (Figure 3b) was indicated by higher WVMR on 17–18, 21, and 27–29 June (Figure 5d). The dry Harmattan northeasterly flow is most pronounced on 22–25 June.
3.3. AI at SS2
[28] The Sahara is bounded to the west by the relatively cool Atlantic and advection of maritime air over the western Saharan by a meso-synoptic-scale circulation; the AI (in [Grams et al. 2010]) is important to maintenance of heat and moisture budgets of both the West African Monsoon Peyrille and Lafore [2007] and the SHL [Lavaysse et al., 2009]. Our observations indicate that SS2 experiences the ef-fects of the AI. During the day, the balance between horizon-tal advection of cool maritime air and turbulence in the CBL over land results in a stationary sea breeze front at the coast. Once turbulence dies down in the evening, the sea breeze front penetrates inland such that the AI is a nocturnal feature over coastal western Saharan [Grams et al., 2010]. Our model simulations (Figure 8a) show that the AI extends over the Atlantic coast of West Africa from ~15N to ~28N (beyond which topography prevents inland penetration) and penetrates many hundreds of km inland by 07:00 UTC the time of maximum extent, with a mean northerly component north of 20N (and therefore at SS2) transitioning to a mean westerly component between 15 and 20N.
[29] At SS2, the morning arrival of the AI can be identified from the 06:00 UTC radiosonde data and surface observations as a cool moist near surface layer in the early morning hours (yellow lines in Figures 4a and 4b). On the basis of the 0000 and 06:00 UTC radiosonde ascents (Figures 4a and 4b and Figure 5d), the AI is apparent on over 60% of days (on some heat low phase days, and on all days in the Maritime phase), as a shallow, cool, moist well-mixed moist layer between the surface and ~100–600 m height a.g.l with WVMR of ~8–13 gkg 1and weak northwesterly winds. The mean depth of the AI layer is 158 m a.g.l., but the AI depth is quite variable (standard deviation = 150 m) with the Maritime phase exhibiting far higher values. Above this, AI layer lies in the LLJ discussed in section 3.4. The structure of the AI in the bottom few hundred meters at 06:00 UTC is clearly distinct from that at 00:00 UTC, when the development of a weak surface temperature inversion below the relatively hot and dry CBL from the preceding afternoon is evident.
[30] At the surface, the AI front (Figure 9) is evidenced by a typically very sharp increase in WVMR (e.g., from 4 to 12 gkg 1 on 13 June,) and wind speed (e.g., from 2 to ~7 ms 1 on 13), a change in wind direction, and drop in temperature (of ~2 K). Subsequently, winds are markedly more turbulent, consistent with a lack of surface decoupling. Arrival time of the AI does vary between about 01:00 UTC and 04:00 UTC, broadly consistent with estimates by Grams et al. [2010] of a mean speed of 10 ms 1over the ~400 km from the coast to SS2 following initiation at ~16:00 UTC. Figure 6. Time series of near surface conditions at SS2. (a)
Temperature (black line) and dew point temperature (green dots) (C). (b) Pressure (hPa). (c) Water vapor mixing ratio (black line, gkg 1) and relative humidity (red line, %). (d) 10-m height wind speed from Fennecflux tower (black line, ms 1) and wind direction from SYNOPS reports (green dots). (e) Radiative fluxes (WM 2); SW down (blue), SW up (cyan), LW down (red), LW up (orange), net radiation (black). (f) SEVIRI cloud fraction at SS2. (g) Aeronet AOD.
[31] Thus, the AI acts to create a rather complex nocturnal structure in the lower layers at SS2. The nocturnal surface temperature inversion, weakly developed by 00:00 UTC (green lines in Figures 4a and 4b), is eroded by the intrusion of the cool shallow well-mixed AI, with implications for the structure of the LLJ described below. As a result, at night after the arrival of the AI, near-surface wind speeds remain relatively high and variable (Figures 6d and 15d), indicative
of turbulence during the night, as a result of the absence of a strong surface temperature inversion and decoupling (yellow lines in Figures 4a and 4b).
[32] A more detailed case study of AI propagation, rather earlier in the day than normal on 17 June is facilitated by analysis of observations roughly along a north-south time-line transect from a dropsonde released some 300 km north of SS2 (during the “heat low survey” flight of the French
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(a) (b) (c) (d) (e) (f) SS2 SS1 SS2 SS2 SS2 SS2 SS2 SS1 SS1 SS1 SS1 SS1Figure 7. Exemplar cloud (red-black color tones) and dust aerosol (pink color tones) conditions over the Sahara during Fennec IOP 2011 from SEVIRI false color dust product. 925 hPa winds from ERA-Interim reanalyses are overlaid. (a) Low aerosol conditions during Maritime phase 12:00 UTC on 3 June. (b) Heat low phase, 21 June 14:00 UTC. Feature A is LLJ-related dust emission from north of SS2, (Feature B is a cold pool over SS1, see Marsham et al., [2012]). (c) Heat low phase, 28 June 15:30 UTC. Features A and B are dust emission from lacustrine deposits. (d) Heat low phase, 20 June 15:30 UTC. Feature A is small-aged cold pool. (e) Heat low phase, 19 June 23:30 UTC. Feature A is small active cold pool; B is aged cold pool. (f) 21 June at 02:00 UTC. Feature A is cold pool from Atlas Mountains. SS1 and SS2 are marked in Figure 7a only.
research aircraft, Figure 9c), the Bir Moghrein Fennec weather station and SS2. The dropsonde at 18.22 UTC shows a well-mixed and rather deep (~800 m), cool (potential tem-peratures ~310 K), and moist (10 gkg 1) layer with wind speeds up to 8 ms 1characteristic of the AI. The inflow front was then evident at the Bir Moghrein weather station as a wind speed and temperature discontinuity at 20:00 UTC (no humidity data available, not shown). Arrival of the AI front at SS2 around 00:00 UTC on 18 June is evident from humidity increase from 6 to 11gkg 1and winds speeds of about 6 ms 1(Figure 9b) and is evident in both the 00:00 UTC and the 06:00 UTC radiosondes as a moist, cool north-westerly layer of only around 100 m depth (Figure 5).
[33] The nocturnal increase in near-surface humidity is apparent on all days except 23 June when strong dry Harmattan northeasterly winds prevailed (Figure 6d). The sharp AI front can be detected on at least nine of the 20 days for which humidity data are available over the full diurnal cycle. On other days, a smooth nocturnal increase
in WVMR occurs, consistent with acceleratedflow towards the SHL across a mean WV gradient from the Atlantic inland. Combined, our surface and radiosonde data suggest that the AI front itself penetrates as far inland as SS2 on at least half of nights during the IOP. The time series of WVMR (Figure 6c) indicates that almost all the moistening of the lower troposphere at SS2 occurs at night, associated with the mean AI and the associated front. Calculation of verti-cally integrated moistureflux in the AI layer, defined as the layer below the temperature inversion, (average height = 158 m a.g.l.) from the 06:00 UTC radiosonde data, indicate a mean value of 8.6 kgm 1s 1over the IOP.
[34] The AI as simulated by WRF (Figure 8) is consistent with these observations at SS2. SS2 is close to the inland limit of AI penetration (Figure 8a), and the magnitude of diurnal cycle in WVMR is at a maximum along a northeast-southwest axis over the western Sahara, passing over SS2, roughly parallel to the coast but around 300 km inland. The spatial extent of the AI indicated in Figure 8 is
SS2
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Figure 8. WRF simulation of Atlantic Inflow feature. (a) June 2011 mean 2 m specific humidity difference (gkg 1) between 07:00 UTC and 15:00 UTC with 10 m winds at 07:00 UTC. (b) June 2011 mean specific humidity (gkg 1) at 07:00 UTC (shaded) and 15:00 UTC (contours) with mean horizontal winds at 7:00 UTC for a north-south transect at 12.5W. (c) Time-latitude plot of June 2011 mean diurnal specific humidity (gkg 1) and 10 m winds for transect at 12.5W.
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Figure 9. Evidence of two observed Atlantic inflow fronts propagating over SS2 and environs. (a) 12–13 June. (b) and (c) 17–18 June. Figures 9a and b show near surface conditions at SS2 where the left Y-axis shows near surface specific humidity (black line, gkg 1) and wind speed (gray line, ms 1), and the right Y-axis shows temperature (red line). Figure 9c shows vertical atmospheric profiles from FF20 dropsonde at 18.22 on 17 June 2011 at 26N,11W, of (left) potential temperature (solid line, K) and relative humidity (dashed line, %), (center) WVMR gkg 1) and (right) wind speed (dashed, ms 1) and direction (solid, degrees). Figure 9d as Figure 9a but from WRF simulation at 3 km horizontal resolution.
further confirmed by evidence of the shallow nocturnal mixed layer in radiosonde ascents from Tindouf (27.6N, 8.1W) in western Algeria (not shown). Figure 8c indicates the propagation of the AI inland from the coast of WA as north-northwesterly low-level flow initiating at ~17:00 UTC and reaching SS2 (22.75N) in the early morning hours, consistent with the observations during at SS2. Comparison of the characteristics of the AI as simulated by WRF at differ-ent resolutions (3 km, 9 km, and other experimdiffer-ents at 27 km) indicates that the sharp AI humidity “front” apparent on many nights at SS2 (e.g., Figures 9a and 9b) can only be rea-sonable simulated at the highest 3 km model resolution (Figure 9d). Analysis of the global ERA-Interim reanalysis product (0.5 degree with 60 model levels) reveals that, despite the rather low 6-hourly resolution of the outputfields, there is evidence of a well-mixed, humid near surface layer characteristics of the AI, which penetrates inland during the night, although at this spatial resolution the sharp AI humidity“front” is not well resolved (not shown). The depth of the AI layer at the location of SS2 at 06 UTC and the asso-ciated moistureflux integrated through the AI layer are also underestimated in ERA-Interim however by ~60%. Thus, the global model captures the essential structure of the AI but struggles to replicate precise detail, perhaps not surprisingly given that SS2 lies close to the inland limit of AI influence. 3.4. LLJ at SS2
[35] Nocturnal LLJs are commonly observed over desert surfaces worldwide (for a review see Stensrud [1996]) and over the Sahara in particular where they are responsible for a substantial proportion of dust emission [Marsham et al., 2012; Schepanski et al., 2009; Todd et al., 2008; Washington and Todd, 2005]. The existence of the LLJ is in essence the result of the diurnal cycle of surface heating and cooling in the context of a pressure gradient. Daytime insolation and sensible heating at the surface creates the hot, dry, and sometimes rather deep CBL observed at SS2. During the night, strong radiative cooling stabilizes the very lowest layers and effectively decouples most of the PBL from surface friction [Figure 4a; Todd et al., 2008] leading to an inertial oscillation around the equilibrium geostrophic wind [Blackadar, 1955; Van de Wiel et al., 2010] with supergeostrophic LLJ winds, which in the Sahara peak in strength before sunrise. After sunrise, surface heating causes the PBL to grow in depth and mixes momentum from the jet level down to the surface creating the distinctive surface peak winds (and dust emissions) from morning to midday observed in many Saharan locations [Marsham et al., 2011, 2012; Todd et al., 2008; Schepanski et al., 2009]. It should be noted that the degree of surface decoupling critically depends on factors suppressing turbulence near the surface during the night. At SS2 (Figures 5d and 5e) the LLJ is clearly evident below ~1000 m a.g.l. on most nights with (northeasterly) wind speeds peaking at 06:00 UTC typically ~6 ms 1 greater than the layer immediately above. However, the characteristic out-of-phase diurnal cycle of LLJ and surface winds is not strongly evident at SS2 (Figures 5d and 6d and section 3.6), although there is a small mean wind speed increase of about 1 ms 1after 06.30 UTC. This apparent paradox of the existence of LLJ without a strong morning peak in surface winds results from the existence of the AI which (i) maintains higher near-surface
wind speeds at night (~5 ms 1) (ii) acts to suppress the decoupling of the surface from the CBL of the previous day leading to a weaker LLJ. As such, the AI undercuts the LLJ, and the LLJ is decoupled not from the surface but from the cooler and shallow AI layer below (see sections 3.4 and 3.6). The LLJ is therefore mixed slowly to the surface, resulting in prolonged moderate peak winds and relatively little dust emission, since dust uplift is a non-linear function of wind speed beyond some surface-dependent threshold.
[36] Day-to-variability in the nocturnal LLJ is, however, pronounced (Figure 5d). During the Maritime phase (6–12 June), the LLJ was a regular but relatively shallow and weak feature extending to about 600 m a.g.l. with maximum speed in the jet core at about 400 m a.g.l. of ~9 ms 1. Subsequently, during the heat low phase, the LLJ was more variable as a result of synoptic-scale variability in regional pressure gradi-ents associated substantially with the evolution of heat low and AEW toughs. On 16, 21–23, and 30, the LLJ was a deeper (core height >1000 m a.g.l) and stronger feature (exceeding>15 ms 1) as a consequence of strong northeast-erlyflow around the surface SHL trough. This strong north-easterly LLJ was also observed to the north of SS2 close to Bir Moghrein on 21 and 22 June from dropsondes released before 0900 UTC (around the time of LLJ break down) from Fennec heat low surveyflights FF22 and B607, respectively, with core wind speeds in excess of 25 and 18 ms 1, respec-tively (Figure 10). In the former case (Figure 10a), the LLJ was likely overlying strong near-surface winds associated with a cold pool gravity current which had spread south from convection over the Atlas Mountains some 500 km to the north during the night. These largest LLJ events generated dust over northern Mauritania (section 3.5). On other days, the LLJ was weaker (14 and 18 June) or absent (28–29), when regional pressure gradients are weaker. Mostly, the LLJ has a meridional northerly component, although on a few days, a southerly LLJ is apparent (e.g., 21 and 25 June), when the influence of southerlies around the surface troughs penetrates as far north as SS2.
[37] The high moistureflux in the LLJ core (Figures 5b, 5d, and 5f), except in cases of the dry Harmattan LLJ, suggests the LLJ plays an important role in the moisture budget of the western Saharan, as it does in the Sahel and southern Sahara [Parker et al., 2005; Lothon et al. 2008]. Calculation of verti-cally integrated moistureflux in the LLJ layer, defined as the layer from the top of the AI to the height of wind speed minima above the LLJ core (average depth of 642 m) from the 06:00 UTC radiosonde data, indicates a mean value of 34.0 kgm 1s 1 over the IOP. These detailed observations provide the bench-marks for evaluation of weather-climate models. As an initial analysis of the performance of global models, ERA-Interim reanalysis data, for example, underestimate the mean noctur-nal peak in low-level moisture and the LLJ core wind speed magnitude by about 30% and 20%, respectively. The bias in ECMWF operation analyses is slightly larger. In detail, although ERA-Interim (and ECMWF operational analyses) reveals a slight weakening of the LLJ between 00:00 and 06:00 UTC at SS2, perhaps as a result of the AI-LLJ interaction (section 3.3), the magnitude of this effect is underestimated and ERA-Interim (and ECMWF operational analyses) actually overestimates the LLJ in the lowest layers 06:00 UTC, whilst underestimating the LLJ during the night (see the mean diurnal cycle of ERA-Interim bias in
Figure 16g, discussed in section 3.6). By way of comparison with other parts of the Sahara, the most extensively studied LLJ feature in the Sahara is that over the Bodele depression in northern Chad. Todd et al. [2008] and Haustein et al. [2012] note that the global NCEP reanalysis and Final Analysis product underestimates the magnitude of by about 50–60% but that higher resolution regional models can pro-vide estimates of peak LLJ wind speeds within ~20%. 3.5. Dust Aerosol Conditions at SS2
[38] Analysis of long-term SYNOP data at Zouerate in a previous study [Klose et al., 2010] reveals relatively frequent reports of dust throughout the year with peak in both the win-ter and summer seasons, indicating that, broadly speaking, Zouerate is influenced by both the winter “Harmattan” dust
process and the summer monsoon trough-related processes. Regarding the latter, dust reports peak in late summer, and June is a month of relatively low dust reports. During June 2011, aerosol loading over SS2 (Figure 6g) and the wider western Saharan domain (Figures 7 and 11) broadly reflects the transition between the Maritime and heat low phases over the Fennec IOP 2011. Aerosol Optical Depth (AOD) re-trievals from the Multi-angle Imaging SpectroRadiometer (MISR) indicate a westward shift in elevated total-column aerosol loadings from the first to the second half of June 2011 (Figure 11). It should be noted that SS2 lies at the periphery of Saharan dust activity during the IOP. Nevertheless, during the Maritime phase, remarkably low AOD values were observed over the western Saharan sectors (Figures 7a and 11a) and specifically at SS2 where AOD
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Figure 10. LLJ features observed from Fennec research aircraft. (a) Vertical profiles of wind speed (blue line, ms 1) and direction (green line) from B607 dropsonde at 09:05 UTC on 22 June 2011 at 24.4N, 10.25W. (b) Vertical profiles of (left) potential temperature (solid line, K) and relative humidity (dashed line, %), (center) WVMR gkg 1) and (right) wind speed (dashed, ms 1) and direction (solid, degrees) from FF22 dropsonde at 07.59 on 21 June 2011 at 26N,11W.
values were persistently<0.2 (Figure 6g). At this time, near the longitude of SS2, the vertical section of Total Attenuated Backscatter from the CALIOP (Figure 12a) shows clear air at Saharan latitudes (~20–27N) and significant aerosol re-stricted to areas south of ~15N, in an elevated layer undercut by the strong maritime northwesterly flow (Figure 2b). Analysis of SYNOP “present weather” reports from the WMO station at Zouerate (only available during daytime) indicates no dust-related codes over the period 1–18 June.
[39] In contrast, during the heat low phase, elevated AOD values (>1) extended over the wider western Saharan sector of Mauritania-Mali (Figure 11b) and at SS2 AOD rose grad-ually from 13 June with peak values of>1.0 on 21 and 23 June and a peak of>2.5 on the afternoon of 30 (Figure 6g). Partitioning local emission and transport from remote
sources at SS2 is problematic, but the SYNOP reports at Zouerate are instructive. During the 12-day period following 19 June, more than half of all present weather reports indicate a dust code. Of these, 73% were code 6 (“widespread dust”) indicative of dust advection from other sources, which were recorded on 19, 21, 23, 25–27, 29, and 30 with visibility <10 km. The time series of these SYNOP reports are consis-tent with day-to-day peaks in AOD (Figure 6g). Weather codes indicating local dust emission were reported on three days (06:00, 09:00 UTC 21 June, with SYNOP westerly winds of 7–8 ms 1; 12:00, 15:00, and 18:00 UTC 23 June, with northeasterly winds of 8–10 ms 1; and at 12:00, 15:00 UTC 29 June, with easterly winds of 8–9 ms 1) all of which are the dust code 7 (raised dust but not a dust/sand storm) except code 32 (dust storm) at 12:00 UTC on 21. Lack of Figure 11. Mean AOD from satellite MISR observations averaged over (a) 1–15 June 2011 and (b) 16–30
Figure 12. (a) and (b) CALIOP lidar backscatter profiles over the west coast of Africa (Saharan sector indicated with a white box) indicative of the“maritime” phase on 3 June 02:46 UTC and the “heat low” phase on 24 June at 02:46 UTC. (c) Lidar backscatter from airborne instrument on FF22 at 08:00–08:30 UTC on 1 June. Dust layers and inferred sources are marked.
high time-resolution surface data at critical times on these days prevents clear analysis of dust emission wind speed thresholds. 23 June is a particularly interesting case with a strong northeasterly Harmattan LLJ and also deep moist convection present over SS2 in the afternoon with gusty winds, suggestive of downdrafts from convection, reaching 15 m/s around midday (the maximum observed at SS2), such that both the LLJ and downdrafts may be responsible for dust emission. Interestingly, occasionally during the Maritime phase of the IOP, winds exceeded values observed on 21 and 29, but no emission was reported (Figure 6d), a paradox that cannot easily be resolved without direct observations of dust emission. There were no reports of the more significant code 9 (dust/sandstorm) at SS2 during the IOP.
[40] Analysis of SEVIRI dust products indicate that ele-vated dust loadings during the heat low phase relate primarily to dust advected over SS2 in the Harmattan low-levelflow from dust sources upstream. Precise identification of these source regions, given the time lags involved in long-range transport and the variety of sources and emission mecha-nisms over the heat low sector (see Marsham et al., [2012]), is a substantial task, which is being investigated in the Fennec project, and we expect to be documented in pub-lication. However, from backtracking plumes in SEVIRI, we can infer that most of the dust observed at SS2 was generated by northeasterly Harmattan LLJ events.
[41] The heat low phase exhibited a strong easterly and northeasterly synoptic-scaleflow over central Algeria, north-ern Mali and Northnorth-ern Mauritania, around the northnorth-ernflank of the heat low and AEW troughs (Figure 3b). Dust emission typically results from elevated surface winds as the nocturnal LLJ mixes to the surface in the mid morning. Dust emission from this mechanism is apparent from SEVIRI data over northern Mauritania on 21, 23 and 28–30 June (Figures 7b, 7c, and 12c), largely explaining the peaks in observed AOD (Figure 6g). The case of 21 June is interesting because the air-borne lidar from Fennecflight FF22 (Figure 12c) shows a near surface dust aerosol layer resulting from a southward moving cold pool which was subsequently augmented by LLJ-related emission later that day (Feature A in Figure 7b). It is interesting to note that the Fennec research
aircraft flights B600 and B601 on 17 June 2011 observed high dust concentrations associated with a similar northeast-erly LLJ event over Northern Mali with exceptionally large size distribution [Ryder et al., 2013], although the trajectory of this dust plume was slightly to the south of SS2. In addi-tion, strong dust plumes were detected from preferential source regions including the palaeo lacustrine deposits at 24.5N, 5.5W, in the extreme Northwest sector of Mali and those at ~26.5N, 1.5E west of In Salah, central Algeria, most notably on 27–29 June (Figure 7c). Rates and precise trajectories of southwestward transport of dust plumes from these sources vary, but typically dust is advected largely to the south of SS2 after dark and so is not apparent in the AERONET observations at SS2. Such dust emission in strong northeasterlies is advected towards the coast of West Africa and is mixed vertically throughout the CBL and into the SRL, exemplified in Figure 12b from 02:46 UTC on 24 June clearly indicating the plume emitted over Mauritania on 23 described above. The contrast in large-scale dust loading at the latitudes of the Sahara between the maritime and heat low modes is evident from comparison of Figures 12a and 12b.
[42] A major mechanism of dust emission during summer-time in the central Sahara is the high wind speeds generated by cold outflows from MCSs. These dust fronts (haboobs), which typically move northwards from nighttime convection over the Sahel (and central Sahara), are known to be respon-sible for a substantial proportion of dust emission in the Southern and central Sahara [Marsham et al., 2012]. Dust from these events can persist over the Sahara for many days following emission and substantially explains the existence of the mean WA aerosol hotspot over the central Sahara in summer [Knippertz and Todd, 2010; Marsham et al., 2008; Figure 1]. However, SS2 is rather too far north and east of the main locus of mesoscale convective activity to be fre-quently influenced by cold pool features from the south. However, convective cold pools influence SS2 in the follow-ing cases; (i) 23 June (see above) when local recorded dust emission is at least in part associated with downdrafts from moist convection over SS2 leading to the maximum observed wind speeds, and due to the non-linear nature of dust uplift
-10.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 06/06 00:00 06/06 12:00 06/07 00:00 06/07 12:00 06/08 00:00 3 ) Fine Coarse 06/20 00:00 06/20 12:00 06/21 00:00 06/21 12:00 06/22 00:00
Figure 13. Aerosol mass concentration derived from LASSfilters collected at Bir Moghrein (ug/m3) for fine (diameter < 2.5 m) and coarse (2.5 m < diameter < 10 m) particles. Left: Days 6 and 7 June (Maritime phase) and Right: Days 20 and 21 June (heat low phase) for coarse (in red) andfine (in blue) filters.
possibly a significant fraction of the local emission during June 2011. (ii) 20 June when an aged cold pool dust plume can be identified at SS2 (Feature A in Figure 7d) whose source was a large haboob initiated over central Mali in the early hours of 19 June. The responsible MCS developed in the southerly monsoon surge around a trough centered over eastern Mauritania (Feature B in Figure 7e).(iii) Night of 20–21 June when a southward propagating haboob from convection over the Atlas Mountains reaches the environs of SS2 late on 21, persisting until morning 22 (Figures 7f and 12c). (iv) Early hours of 20 June a much smaller cold pool feature was observed propagating from convection over coastal Southern Morocco on the night of 19–20 June arriv-ing at SS2 in the (Figure 7e), but with limited dust loadarriv-ings. [43] Results from in situ sampling of dust collected by the LASS in Bir Moghrein highlight the contrast between the Maritime phase and heat low phase. Figure 13 shows the mass concentration of aerosol particles in units ofmg/m3of air, collected on days 6 and 7 June (Maritime phase) and on days 20 and 21 June (heat low phase). Concentrations were calculated based on the mass collected on the Nucleporefilters and on the volume of air sampled during every 6 h period starting at 07:00 UTC. Each value in the plot is associated with the initial time of sampling, although the concentration repre-sents an average for the whole 6 h period of sampling. During the Maritime phase period, very clean conditions were ob-served, with fine and coarse mode concentrations below 10mg/m3 and close to the detection limit or our technique, except for the morning of 7 June where this concentration is slightly larger. During the heat low phase, the observed con-centrations vary from 5 to 70mg/m3with oscillations in the predominance between thefine and coarse modes along this period. Figure 14 shows two Scanning Electron Microscopy (SEM) images of afine and a coarse filter collected between 19.00 UTC on 20 and 01.00 UTC on 21 June. SEM images show in black the circular pores of the filter, with particles collected on its surface on a lighter color. Coarsefilters have pores of 5 um, and they were used to retain particles with size mostly smaller than 10 um. Fine filters, positioned after the coarse filter, were used to collect particles with size mostly below 5 um. Figure 14 shows asymmetrical particles with a large variety of shapes. Further image analysis will be applied to obtain particles size distribution.
[44] Estimating the effect of dust (and cloud) on the radia-tion budget is compromised by a lack of downwelling short-wave flux data in the latter days of the IOP (Figure 6d).
During the heat low phase, a complete set of daytime data is only available on 26, with cloud-free conditions. On this day, observed AOD of ~0.5 coincided with peak downwelling shortwaveflux values about 100 Wm 2lower than the days during the Maritime phase, (differences due to solar zenith are marginal at this time of year). Therefore, we make an estimate that enhanced aerosol loadings in flu-ence the surface radiation budget such that downwelling shortwavefluxes decline by ~200 Wm 2per unit AOD, com-parable to that at SS1 where analysis was further complicated by cloud [Marsham et al., 2012]. This is more than thefigure of 140 Wm 2per unit AOD during the major dust event of Figure 14. SEM images of Nucleporefilters with fine (left) and coarse (right) aerosol particles collected
using the LASS instrument at Bir Moghrein during the period of 19:00–01:00 UTC between 20 and 21 June.
March 2006 [Slingo et al., 2006] For this case, Milton et al. [2008] demonstrate that the inclusion of dust aerosol pro-cesses in a Numerical Weather Prediction reduced the large model bias in radiationfluxes considerably. Similarly, analy-ses of cloud radiative effects are problematic due to very limited radiation data in the heat low phase.
3.6. The Mean Diurnal Cycle
[45] Mean diurnal cycles (derived from all available data during June 2011) at the surface and through the troposphere are shown in Figures 15 and 16, respectively. The mean diur-nal cycle in many surface variables is strongly driven by the surface energy balance (Figure 15). Mean net radiation peaks at ~400 Wm 2during the day and is around 100 Wm 2at night, indicating a very large diurnal cycle of radiative
heating and cooling. Net LW cooling varies from around 100 Wm 2 at night to ~290 WM 2 during daytime. Upward SWflux is around 31% of downward SW flux such that net SW heating peaks at almost 700 Wm 2at local mid-day. Upward LW flux peaks at ~13:30 UTC, leading 2-m atmospheric temperature by ~2 h. Downward LWflux shows a maximum (minimum) at around 18:00 (06:00) UTC, reflecting the temperature of the overlying atmosphere and the diurnal cycle in cloud cover.
[46] The strong diurnal cycle in heating and cooling leads to a pronounced diurnal cycle in 2-m temperature with a maximum of ~36C around 16:00 UTC and a minimum of ~24C around 06:00 UTC (Figure 13a). The classic semi-diurnal cycle in surface pressure [Pugh, 1987] is apparent (Figure 15b). Unlike many other Saharan locations, there is Figure 16. Mean diurnal cycle at SS2 of the vertical profile of (a) potential temperature (K), (b) WVMR
(gkg 1), (c) relative humidity (%), (d) wind speed (ms 1) from radiosondes (shaded) and pibal (contours) observations, (e) wind direction (degrees from north), (f) WVMR flux (gkg 1ms 1), (g) bias in ERA-Interim reanalysis wind speed (shaded, ms 1) and WVMR ( 2 gkg 1contour), with respect to radiosonde data (ERA-Interim is interpolated to 1 hPa in the vertical). Note different height ranges in Figures 16d–g compared to Figures 16a–c.
a relatively weak diurnal cycle of near-surface wind speed (Figure 15d). We interpret this as result of the lack of surface decoupling at night due to the AI (see section 3.4) which maintains relatively high nighttime surface wind speeds, limiting the strength of the LLJ at night (section 3.5) and providing a strong temperature inversion above the AI and below the LLJ, such that downward mixing of the LLJ in the hours after sunrise is relatively weak. The diurnal peak in near-surface winds at 19:00 UTC exists both in observa-tions (Figure 15d) and in model simulation (not shown), at a time when wind speeds associated with turbulent eddies in the afternoon (see Figure 15d) would normally be expected to decrease, e.g., Todd et al. [2008]. We propose an explanation of this rather unusual diurnal cycle in surface winds from interpretation of the radiosonde and Pibal winds below.
[47] Radiosonde data are limited to the 00:00–14:00 UTC period so that we do not completely sample the diurnal evolution of vertical mixing in the SABL. Profiles indicate that overnight cooling occurs below about 2000 m (Figure 16a) with an increase in low-level wind speed associated with the nocturnal northeasterly LLJ (Figure 16d) peaking at>10 ms 1 at 00:00 UTC at about 400 m above the surface. The classic clockwise rotation over the diurnal cycle of the zonal and meridional components of wind in the LLJ core (and surface winds) noted in other studies [Todd et al., 2008; Marsham et al., 2012] is apparent here and confirmed from the WRF simulation (not shown). The LLJ peak at SS2 is, however, somewhat earlier and weaker than LLJs observed elsewhere, which peak just before sunrise. It seems that the weak morning peak in LLJ at SS2 results from the arrival of the AI in the early hours of the morning (mean northwesterly near surface flow at 06:00 UTC flow in Figure 16e), which acts to mix near surface and lowest atmospheric layers (e.g., yellow line in 4a) and to limit decoupling and further acceleration of the noctur-nal LLJ (see also section 3.4), a process which may be under-represented in the ERA-Interim reanalysis (and ECMWF operational analyses) data, as indicated by the overestimation of the LLJ by these models in the lowest layers (Figure 16g). Indeed, the height of the LLJ in observations actually rises slightly through the early hours as the AI layer deepens, a feature not captured by the models. The LLJ decays from the surface upwards after sunrise as the depth of the CBL deepens and turbulent mixing erodes the LLJ. Pibal data (contours in Figure 16d) indicate that the nocturnal LLJ actually persists at ~1300 m at 09:00 UTC, above the growing CBL. The CBL growth starts later at SS2 than other more continental locations due to its western position, and the growth rate slower due to the existence of the cool AI air, such that the LLJ decays most rapidly between 09:00 and 12:00 UTC. In the early afternoon, the growing CBL breaks through into this now elevated LLJ and mixes momentum to the surface maintaining elevated surface wind speeds through the afternoon (Figure 15d). As the CBL grows through the lower few km of the atmosphere, momentum from strong winds (~8–9 ms 1) at heights of 2500–5000 m appears to be mixed downwards to the surface (note the decline in wind speeds at ~2000 m and the downward sloping wind speed contours below ~1000 m from pibals). This probably explains the peak in surface winds speeds at ~18:00 UTC, although we cannot rule out the effect of local topographic and albedo features.
[48] Regarding moisture, nocturnal mean moistening is evi-dent at the surface (Figure 15c, the WVMR peak values of>9 gkg 1occur at ~07:00 UTC and minimum values of ~4 gkg 1 at ~19:00 UTC) and in the lower layers (Figure 16b increases in WVMR from ~6 to ~9 gkg 1from 00:00 to 06:00 UTC). We associate this cycle with both the mesoscale AI circulation and the nocturnal acceleration offlow towards the continen-tal interior across the mean moisture gradient towards the central Sahara, described in more detail in section 3.3. The ERA-Interim (and ECMWF operational analyses, not shown) underestimates water vapor in the lowest layers by about 20–30% as shown in Figure 16g. There is an interest-ing distinction between the Maritime phase and heat low phase (not shown) in that the former experiences a rather deeper and uniform moist layer of some 500 m depth which may be indicative of the depth of the AI in this period.
[49] At SS2, cloud cover (Figure 15f), especially during the heat low phase, exhibits a strong diurnal cycle with gen-erally cloud-free conditions through the morning and early afternoon, but rising sharply at ~15:00 UTC and peaking at ~20:00 UTC at which time mean cloud cover is 75%. This is associated with the development of shallow convection at the top of the SRL during days when mixing of water vapor occurs throughout the depth of the SRL/CBL and high rela-tive humidity values at the top of the SRL are maximized.
4. Summary and Conclusions
[50] Lack of observations over the central Sahara sector has hindered our understanding of the dynamical and ther-modynamical processes in this large region, and therefore both climate and NWP model skill is often limited in this region and beyond. This is particularly important during the boreal summer months when the SHL and aerosol maxima are most fully developed and interactions with the West African Monsoon most significant. The project “Fennec: the Saharan climate system” sought to fill gaps in our obser-vations with a comprehensive set of surface and upper air ob-servations from ground and aircraft-based instrumentation. This paper summarises the keyfindings from ground-based observations at Fennec SS2 at Zouerate, Mauritania in the Western Sahara during the IOP of June 2011. We are able to quantify and provide uniquelyfine detail on some of the key processes associated with diurnal to seasonal-scale evo-lution of the western Saharan atmosphere, notably ventilation and humidification of the western Saharan region and the SHL associated with a range of meso- and synoptic-scale processes.
[51] During the IOP of June 2011, SS2 and the western Saharan region experienced a rapid transition around 14 June from what we refer to as a Maritime phase to a heat low phase with markedly contrasting atmospheric and aero-sol conditions. The Maritime phase is characterized by a SHL displaced to the east of the mean June position, such that the western Saharan region experiences a strong regional-scale maritime influence, driving large-scale cold advection throughout the lowest ~4 km of the troposphere into a rather large swath of the western Saharan region. This maritime air has very low aerosol loadings. In contrast, during the heat low phase, the SHL is shifted westward to lie close to or westward of its June climatological position and the western Saharan region is essentially hot (>10 K warmer than the
